2015 Model S 85D owner here. I get this noise if I've charged my car above about 78% SOC or so. I believe it's a coolant pump related to the battery pack. This started happening around the time that Tesla changed some of the battery management system programming in 2019 (see "batterygate" and "chargegate").
To the best of my knowledge the noise is harmless although I've taken to charging to 75% for daily usage just to reduce wear and tear on whatever mechanisms are running.
Bruce.